Automatic volume control for telephone recording apparatuses



Jan. 25, 1944. THOMPSON 2,340,159

AUTOMATIC VOLUME CONTROL FOR TELEPHONE RECORDING APPARATUS.

Filed Nov. 21, 1942 3 SheetsSheet 1 INVENTVOR LINCOLN THOMPSON BY ATTORNEY L. THOMPSON Jan. 25, 1944. I

AUTOMATIC VOLUME CONTROL FOR TELEPHONE RECORDING APPARATUS Filed Nov. '21, 1942 5 Sheets-Sheet 2 JHHUHI I I I H P HH H I IHHU I HH uH h lH HJHHH I PNWII 1 l I|l II '"l. l

' INVENTOR LINCOLN T HQMPSON ATTORNEY Jan. 25, 1944.

L. THOMPSON AUTOMATIC VOLUME CONTROL FOR TELEPHONE RECORDING APPARATUS Filed NOV. 21, 1942 .3 Sheets-Sheet 5 re-am alifler "VA." l 97 .98 23/ 942 y 42$" 77/ k TJ.:\..-. 79 6? 8/ INVENTOR LINCOLN THOMPSON BY I A TTORNEY Patented Jan. 25, 1944 I ferric-s PHONE RECORDING APPARATUSES 'Llncoln Thompson, Cheshire, Conn.. minor to The SoundScrlber Corporation, New Haven, Conn., a corporation of Connecticut Application November 21, 1942, Serial No. 466,485

(or na-o) 5 Claims.

This invention relates to electrical phonograph recorders, and more particularly to an automatic volume control system for recording conferences and telephone line.

One object of thepresent invention is to provide an automatic volume control unit and apparatus of-the above-nature designed to be two-way conversations in a .connected to the terminals of atelephone receiver in such a manner that the installation will in no way afiect or impair the telephone transmission or absorb any appreciable amountof current energy from the telephone circuits, thus avoiding any possible service problems.

1 Another object is to provide an automatic volume control apparatus of the above nature designed to be inserted between the telephone line or other circuit and the input of the amplifier oi a phonograph recorder.

A further, object is to provide an apparatus of the above nature in which the high intensity oi" the local voice currents is cut down or attenu' ated so as to reduce the contrast with the weaker distant voice currents and thus permit clearer recording. A further object is to provide an apparatus of the above nature housed, in a suitable casin and including a shunting copper oxide rectiher, a "biasing copper oxide rectifier, avolumel control knob, and a selector switch to permit selective recording Irom a microphone or from the telephone line. f

A further object is to provide a simple attachment of the abovenature which may be connected to an individual recording uniton the user's desk to provide this automatic volume con-. trol feature as an external addition to the recording apparatus without need-for connection to a power line or to batteries.

. A further object is to provide an automatic volume control apparatus and system of the I above nature which will be simple in construction, inexpensive to manufacture, easy to install and connect at both ends 'of the telephone line, easy to manipulate, compact, and very convenient, eificient, and durable in use.

With these and other objects in view there have been illustrated on the accompanying drawings several forms in which the invention may be conveniently embodied in practice.

In the drawings:

Figure 1 represents a plan view of the phonograph recording cabinet, the volume control casing, and the various electrical plugs and connector cords employed therewith.

Figure 2 is a circuit diagram of the same. Figure 3-is a circuit diagram ofa modified form of the invention employing a-pair of "shunting rectiflers in push-pull relationship.

Figure 4 is a circuit diagram of another modifled form of hookup also embodying the'inven tion and in which a preamplifler" is employed. The present invention is designed to couple an ordinary telephone line with a standard type of phonograph recorder, such as disclosed in the Patent No. 2,250,242 to Lincoln Thompsomdated July 22, 1941, and entitled Disk dictatin machine.

The purpose of the present invention is to permitthe faithful recording oi a two-way ielephone conversation or other source or signals in which weak signals from a distant telephone or other weak source are combined with stronger signals from a nearby transmitter or'froinanother strong source.

This is accomplished by providing an improved automatic volume control unit which wills-substantially balance the eflects of the distanhand local voices of the telephone conversation.

In addition to the above function orbalancing the loud and soft voices, the present systemalso reduces the level of thesignals lromthe-entire telephone line which: is too high for use -with the'recording apparatus mentioned above, so

that a much lower signal level will be impressed.

upon the input of said apparatus, In addition. it reduces the volume of the high level" signals much more'than it does the volume of the low level signals on the line; I Theory oj opefa tion The improved automatic volume controlapparatus of the present invention depends for its operation upon the shunting eilect of a copper oxide rectifier across a part of the signal input circuit, which is so arranged thata variation-in the shunting impedance at the point or :application or said rectifier will produce a substan-- tial change in the signal voltage applied to'theinput of the recorder amplifier.

'In'actualuse, the telephone line will be connected to a transformer primary through aypair 4 of condensers and a pair of resistances, aridthe secondary of said transformer is included in a circuit having a copper oxide rectifier in shunt therewith. I

When the copper oxide shunt rectifier ;provides a load on the secondary of the transformer it reflects back tothe primary of the transformer and introduces a voltage drop by virtue oi-the transformer losses and the resistances and con densers connected inseries with the telephone line.-

The variation of the shunting impedance of this rectifier is produced by varying the D, C.

biasing voltages applied through a second copper oxide rectifier which rectifles a portion or the -output signals from the recorder amplifier.

These rectified signals are smoothed by is condenser-resistance network and appear as direct current biasing voltage in series with the shunting copper oxide rectifier. A strong telephone signal will produce a strong-Del). biasing voltage and thereby cause the shunting rectifier to produce a large amount of shunting effect. A weak telephone signal on the other hand'will produce very little 1). C. biasing voltage, and the shuntlng effect of the first copper oxide rectifier will therefore be very low for such signals,

Thus, for weak signals, the shunting copper oxide rectifier will produce only a small reduction in the signals entering the recorder amplifier, and for strong signals it will produce a decided reduction.

' Description Referring now to the drawings in which like reference numerals denote corresponding parts throughout the several views, the numeral 10 indicates a phonograph recorder of the type disclosed in the Thompson Patent No. 2,250,242 mentioned above, designed for recording dictation and other sounds on a rotating disk-record ll.

. The recorder cabinet is provided with a control knob l2 for adjusting the gain of the amplifier (not shown) employed in connection therewith. The record is driven by a motor (not shown) under the control of a lever switch l2a, said motor being energized by a power cord l3 having a twoprong plug 14 for connection with any convenient electric power socket.

The amplifier of the recorder I0 is adapted to- 2), forming part of a closed circuit including a pair of high resistors 20, 2| (which may be of the order of 1,000 ohms), a pair of condensers 22, 23 (which may be of the order of 1 microfarad) and which serve to thoroughly isolate the telephone line from the automatic volume control unit and the recording apparatus. The condensers 22', 23 are joined by conductors 24, 25 to the primary coil 26 of a transformer 21 having a secondary coil 28.

Connected to the secondary coil 28 of the transformer 21 are a pair of conductors 25, 80, the

former having a high resistance 8| (of the order of 1250 ohms) in series therewith and which leads through a wire 32 to a variable resistance 33. An arm 34 manually controlled by .a pointer knob 35 (Fig l) is in electrical contactwith said resistance 33 and leads to. one "telephone" terminal 36 of a double-throw double-pole switch arm 3] which serves for selectively connecting I the telephone line with a pair of central switch terminals 38, 39 leading through wires 40, 4| to the outer terminals 42, 43 of a four-prong plug for insertion into a, socket (not shown) on the recorder amplifier.

The conductor 88 is connected to a cross wire 44 having a condenser 45 (about 80 microfarads) located therein; which in tumleads through conductors 48, 41 to the other telephone" terminal I i 48 of the switch 81. The arm 84 is connected to the conductor 41 by a conductor 480. which includes a resistance 48b.

The switch 31, when swung to the opposite position from that shown on the drawings, is adapted 'to engage a pair of terminals 49, 50 which are connected by lead wires 81, 82 to the-outer terminals of a four-prongfmicrophone socket,"

connected by a cable 84 to a microphone for use when a recording is to be made of a conference or ordinary dictation.

The two centralterminalsifi, 5! of the ampliher are joined by wires'58, 59, and .60, 6| to the central terminals 62, 68 respectively of the fourprong microphone socket. Provision is also made of a conductor wire 64 leading from the Junction of wires 58, 58 to the core 65 of the transformer, said core being connected to an electrostatic shield in the transformer between the primary and secondary windings.

Th present automatic volume control unit isprovided with a pair of copper oxide rectifiers 68, 61, the former of which is connected in shunt with the transformer secondary 28' by means of a wire lit-and a wire 69 leading to the junction 18 between the conductors 46, 4 1

The second rectifier 6'! is connected to a twoprong plug 1| leading from the output of the recorder amplifier through a pair of conductors 12 I3 (as shown in Figure 2). The wire I2 leads by a wire 14 to the junction 10,-while the wire 13 leads through a small resistance 15 (25 ohms) to a wire 16 joined to one side of the rectifier 61.

The other side of said rectifier 61 is joined by a wire I1 and aresistance I8 (about 200 ohms) to the wire 30.

V In order to filter the amplifier output as well as to rectify it, provision is made of a second condenser 19 (about microfarads) whichco operates with the resistance 15, the resistance I8, the condenser 45 and a resistance 80, the latter being joined to the wires '14 and 11 as shown in Figure 2. If desired an additional condenser 8i may be connected to the wires 14 and 11 between i the resistance 80 and the rectifier 61 in cases where there are an excessive amount of high frequency signals passing through the amplifier, and which tend to exert an inordinate control over the circuit.

'- Operation In operation, the condenser 8! serves to eliminate the effect of such high frequency signals and limits the volume control action to the lower frequency signals which occur in ordinary voice recording, This insures that the automatic volume control will be operative only on the lower frequency speech signals and that the high-pitch portion of the energy will take no part in said control. The automatic volume control unite! the present invention may be fastened to the back of a standard recorder cabinet by means of screws 82, as shown in Figure l..' The spade terminals l5, 16 will then be attached to suitable terminals on the telephone equipment. After ad-' justing the control knobs i 2 and 35, the apparatus will be ready for use. 1

It will be understood that by connecting the copper-oxide rectifier 61 to the output of the recorder amplifier for rectifying a portion of said output and filtering it through a network of smoothing condensers and a resistor, a D. C. 4 bias will be provided free from signal ripples. A time delay will also be introduced which may be varied by changing the series resistances or the filter condenser capacity. The control of this time delay is necessary to avoid distortion and choppiness which would result from an instantaneous action, or too rapid an action between syllables of aword. Moreover, too slow an action will cause loss of the first parts of a weak voice following a strong one.

It will be understood that the amount of bias of the second rectifier 61 changes with the output signals and this in turn changes the impedance of the shunting rectifier unit.

Modified form In the modified form of the invention illustrated in Figure 3, a pair of shunting rectifiers B3, 84 arranged in "push-pull relation are employed in place of the simple shunting rectifier 66, shown in Figure 2.

The rectifiers 83, 84 serve to balance out any unrectified signals which have passed through thefilter system. By the use of this push-pull arrangement, much less filtering is necessary, or in other words, the condensers may be made of smaller capacity. A further advantage due to the elimination of the unrectified' signals lies in the fact that regenerative effects are entirely avoided.

The rectifiers 83, 84 are connected to the conductors 3|, an by wires 85, 85 respectively, and are connected together by an intermediate conductor 81, the latter being joined at its midpoint 88 to a conductor 89 leading to the conductor 14. In tnis form of the invention, the secondary 28 is tapped at its midpoint so and connected to the conductor H by a conductor 9|. The condenser 45 in this instance is connected to the conductors t9 and 9 I, instead of to the conductors l4 and I1 as in the first form of the invention.

In the modified form or hookup shown in Figure 4, the telephone line is applied to a vacuum tube preamplmef' 92, whose output isconnected by conductors s3, 94 to a pair of resistors 35, as

'constitutmg a voltage divider. A rectifier 81 is in a shunt circuit including the resistor so.

condenser 45 is of high capacity and acts for all practical purposes like a solid connection for A. C. between the unctions s8 and so. In this form of the invention, the biasing rectifier 61, the condensers 45,18, and BI and the resistances l and an all perform the same functions as in the first two forms of the invention.

The

be embodied, it is to be understood that these forms are shown for the purpose of illustration only, and that the invention is not to be limited to the specific disclosures, but may be modified and embodied in various other forms without departing from its spirit. In short, the invention includes all the modifications and embodiments coming within the scope of the following claims.

Having thus fully described the invention, what is claimed as'new and for which it is desired to secure Letters Patent is:

1. In a gain control system, a high resistance primary circuit connected to the terminals of a telephone line, a secondary circuit transformer coupled to said primary circuit, means to connect said secondary circuit'to the input of a recorder amplifier, a copper oxide rectifier shunted across said secondary circuit, and a second copper oxide rectifier energized by a portion of the output from said amplifier to vary the load of said shunting rectifier in accordance with the strength of the signals in said telephone line.

2. In a system for recording telephone conversations from local and distant points, a high resistance primary circuit connected to the terminals'of a telephone line, a secondary circuit coupled to said primary circuit and having a variable resistance therein, a recorder amplifier, a switch for connecting said secondary circuit to the input of said amplifier, a rectifier shunted across said secondary circuit, and means for impressing a portion of the output from said amplifier upon said rectifier to cause the impedance of said rectifier to vary inversely with the input energy and thus maintain a uniform amplifier gain.

3. In a system for selectively recording telephone conversations and signals received from a microphone, a high resistance primary circuit connected to the terminals of a telephone line, a secondary circuit coupled to said primary circuit, a microphone circuit, a recorder amplifier circuit, and means for selectively connecting said One advantage of the modified form shown in Figure 4 is that the rectifier Bl acts as a low resistance shunt for strong signals across the resistor 86. It also permits a larger range of volume control due to the preamplifier, and reduces the level of undesired amplifier noises arising from sources other than the voice or other sounds to be recorded.

An important feature of the present invention lies in the fact that the improved volume control apparatus is used with a circuit which has inherently a tremendous amount of attenuation. In other words, in order to function within the range required, a considerable amount of loss has to be introduced in order to adapt the telephone line voltages to the voltages permissible for the operation of the recorder amplifier.

One advantage of the present invention is that it provides a highly convenient unit for attachment to a recorder amplifier without requiring any changes to be made in the amplifier circuit and without the necessity of the use of vacuum tubes, batteries, or other energy consuming devices.

While there have been disclosed in this specifi cation several forms in which the invention may sations from local and distant points, a pre-amplifier connected to the terminals of a telephone line, a high resistance circuit connected with said pre-amplifier and including a rectifier in shunt therewith, a recorder amplifier, and means for filtering and impressing a portion of the output from said recorder amplifier upon said secondary circuit to vary the shunting load of said rectifier inaccordance with the strength of the'applied energy whereby a uniform volume of amplification will be produced and recorded.

5. In a system for recording telephone conversations from local and distant points, a high resistance primary circuit connected to the terminals of a telephone line, a secondary circuit coupled to said primary circuit, a pair of rectifiers in push-pull arrangement shunted across said secondary circuit, a recording amplifier connected 

